The geminiviruses are a large and diverse family of plant DNA viruses, with circular single-stranded (ss) DNA genomes that replicate through circular double stranded DNA intermediates. See Hanley-Bowdoin et al., Cri. Rev. Plant Sci. 18:71 (1999); Lazarowitz, Crit. Rev. Plant Sci. 11:327 (1992); Timmermans et al., Annu. Rev. Plant Physiol. 45:79 (1994). Viral DNA replication, which results in both single and double stranded viral DNAs in large amounts, involves the expression of only a small number of viral proteins that are involved in either replication or viral transcription. The geminiviruses appear to rely primarily on the machinery of the host to copy their genomes and express their genes, including the nuclear DNA and RNA polymerases of their plant hosts. These properties of geminiviruses are unusual among plant viruses, most of which are RNA viruses or replicate through RNA intermediates using virus-encoded replicases. Geminiviruses infect a broad variety of plants and cause significant crop losses worldwide.
Geminiviruses are subdivided on the basis of host range in either monocots or dicots, genome structure, and insect vector. Subgroup I geminiviruses (also known as Mastreviruses) are transmitted by leafhoppers and infect primarily monocots, although Subgroup I geminiviruses that infect dicots are known. Subgroup II geminiviruses (also known as Curtoviruses) are transmitted by leafhoppers and infect dicots. Subgroup III geminiviruses (also known as Begomoviruses) are transmitted by whiteflys and infect dicots. Subgroup I & II viruses have genomes comprising a single ssDNA component; Subgroup III geminiviruses typically have a bipartite genome comprising two similarly sized DNAs (usually termed A and B), as illustrated by African cassava mosaic virus (ACMV), tomato golden mosaic virus (TGMV) and potato yellow mosaic virus. However, monopartite geminiviruses that infect dicots are known, for example Tomato Yellow Leaf Curl Virus (TYLCV). The genomes of monopartite Subgroup II and III geminiviruses have an arrangement of genes similar to the AL1, AL2 and AL3 genes found on the A DNA component of bipartite Subgroup III geminiviruses.
Subgroup III viruses are also divided into “old world” and “new world” viruses, a division based on evolutionary divergence.
For successful infection of plants by bipartite geminiviruses, both the A and B genomic components are required. Sequence analysis of the two genome components reveals six open reading frames (ORFs). Four of the ORFs are encoded by DNA A and two by DNA B. On both components, the ORFs diverge from a conserved 230 nucleotide intergenic region (common region) and are transcribed bidirectionally from double stranded replicative form DNA. The ORFs are named according to genome component and orientation relative to the common region (i.e., left versus right (L/R), or virion versus complementary sense (V/C)). Certain proteins are known to be involved in the replication of viral DNA (REP genes). See, e.g., Elmer et al., Nucleic Acids Res. 16:7043 (1988); Hatta and Francki, Virology 92:428 (1979).
The A genome component contains all viral information necessary for the replication and encapsidation of viral DNA, while the B component encodes functions required for movement of the virus through the infected plant. The DNA A component of these viruses is capable of autonomous replication in plant cells in the absence of DNA B when inserted as a greater than full length copy into the genome of plant cells, or when a copy is transiently introduced into plant cells. In monopartite geminivirus genomes, the single genomic component contains all viral information necessary for replication, encapsidation, and movement of the virus.
Geminiviruses cause substantial losses among economically important crops, including tomato, bean and cucurbit. Current strategies to control geminivirus infections target the insect vectors that carry the viruses. However, the use of insecticides to control or combat a geminivirus infection can be expensive and inefficient. Additionally, insect hosts can vary in their susceptibility to available insecticides, and resistance to insecticides can develop over time. See Markham et al., Pestic. Sci. 42:123 (1994).
Varied approaches have been used in attempts to generate geminivirus-resistant plants, including classical breeding and transgenic approaches, with limited success. Unlike plant RNA viruses, the introduction of geminivirus sequences into transgenic plants does not confer resistance, and conversely, frequently results in the production of functional viral proteins (Hayes and Buck, Nucleic Acids Res. 17:10213 (1989); Hanley-Bowdoin et al., Proc. Natl. Acad. Sci. USA 87:1446 (1990)). Kunik et al. describes transgenic tomatoes that contain a geminivirus coat protein gene (Kunik et al., BioTechnology 12:500 (1994)). Expression of antisense RNAs against geminivirus replication proteins in transgenic plants reduces the level of viral DNA accumulation up to 70% (Day et al., Proc. Natl. Acad. Sci. USA 88:6721 (1991)), to a level that is still sufficient to confer wild type viral symptoms (Hanley-Bowdoin et al., Plant Cell 1:1057 (1989)). Similarly, the presence of defective-interfering replicons in transformed plants can reduce the level of viral DNA accumulation by about 70% (Frischmuth and Stanley, Virology 200:826 (1994)). The antisense RNAs and defective-interfering replicons function best against their cognate viruses (Bejarano et al., Plant Mol. Biol. 24:241 (1994)), further limiting their usefulness. Antisense RNA targeted to mRNA of the Rep protein (encoded by the C1 gene) was used to produce transgenic Nicotiana benthamiana plants with altered responses to TYLCV. (Bendahmane and Gronenborn, Plant Mol. Biol. 33:351 (1997)).
Accordingly, it is desirable to devise new strategies to control geminivirus infection.